Semiconductor devices are currently requiring more levels of interconnects as component densities within the devices increase. A conventional local oxidation of silicon (LOCOS) field isolation process may cause problems because the field isolation regions may have too much of a step height change that may make subsequent planarization steps required or more difficult. Other field isolation processes that are similar to LOCOS, such as sealed-interface local oxidation (SILO), poly buffered LOCOS (PBL), LOCOS processes having silicon or nitride spacers, and the like, have similar step height problems.
LOCOS-type field isolation processes may cause problems when subsequently patterning a layer to form a gate electrode because reflective notching may occur. The changes in topography between a substrate and LOCOS-type of field isolation regions cause light to reflect and form notches in a masking layer pattern used to form a gate electrode. The mask pattern may have a feature with a uniform width, but the pattern formed in the masking layer by that mask may have a width that narrows near the edge of a field isolation region because of the reflective notching. The pattern in the masking layer is transferred to the conductive layer when the conductive layer is etched to form the gate electrode. This differenge between the pattern of the mask and the gate electrode is undesired.
Other field isolation processes have been developed that require etching a semiconductor substrate. These other processes, including trench isolation, semi-recessed or fully-recessed LOCOS processes, sidewall-masked isolation (SWAMI), and the like, all require some type of substrate etch. Trenches in the substrate may not have uniform depths. Narrow trenches may not be formed to the same depth as wider trenches because the concentration of ions (of a plasma) is typically lower for the narrower trench. Therefore, narrow trenches will be shallower than wider trenches. This effect is called "lag." Another problem with a substrate etch is that typically an endpoint does not exist. The depth of the substrate trench may be variable both across a substrate and between substrates of the same lot. A lack of control for the substrate etch poses a problem for which no good solution exists.